X The Troxler effect: the fading of a large stimulus with blurred edges presented in the peripheral visual field

Chapter 7

Temporal Factors in Vision

Spatial vision is not possible

unless the retinal image changes

with time

Spatial vision is not possible unless the retinal image changes over time

X

The Troxler effect: the fading of a large stimulus with blurred edges presented in the peripheral visual field

Five Parts to this Chapter

Temporal Acuity (critical flicker frequency [CFF])

The Temporal Contrast Sensitivity Function

Temporal Summation

Masking

Motion Detection (Real and Apparent)

Part One

Temporal Acuity:

The critical flicker frequency (CFF)

The critical flicker frequency (CFF) is a measure of the minimum temporal interval

that can be resolved by the visual system.

CFF is analogous to grating acuity as a measure of spatial resolution acuity

Measure CFF using an episcotister

(a rotating sectored disk used to produce square-wave flickering stimuli)

the period is the length of time for one complete cycle of light and dark, and the flicker rate,or flicker frequency is the number of cycles per second (Hz)

Duty cycle – the ratio of the time a temporal square-wave pattern is at Lmax to the time it is at Lmin

Talbot brightness = Lmin + ([Lmax – Lmin] x f) Eq. 7.1

The time-averaged luminance of a flickering light determines its brightness at

flicker rates above the CFF (Talbot-Plateau Law)

where f is the fraction of time that Lmax is present during the total period

How bright does a fused flickering light appear?

To convert duty cycle to f, divide the first number by the sum of the two numbers: 1:1 means f=0.5

If a square-wave flickering light has a duty cycle of 4:1, what is f?

.1 .2 .4 .8 1.

0% 0% 0%0%0%

1. 0.1

2. 0.2

3. 0.4

4. 0.8

5. 1.0

To convert duty cycle to f, divide the first number by the sum of the two numbers: 4:1 would be 4/(4+1) so f=0.8

How bright does a flickering light appear?

At flicker rates slightly below the CFF, brightness is enhanced beyond the mean luminance of the flicker (the Brücke-Bartley phenomenon)

This is related to the Broca-Sulzer effect described later in the chapter

Time

Stimulus luminance

Neural response

Stimulus luminance

Neural response

Stimulus luminance

Neural response

A

B

C

The neural basis of the CFF is the modulation of firing rates of retinal neurons (ganglion cells)

Courtesy of Dr. Tim Kraft

Cone flicker response (pig). Contrast 0.49; mean light level 48,300 photon/ square micron

Rat ganglion cell responses showing CFF

In order to see a light as flickering

The

flick

er ra

te m

ust ..

The

Troxl

er e

ffect

mu..

Ret

inal

neu

rons

must

..

All

of the

other

answ

e..

0% 0%0%0%

1. The flicker rate must be above the CFF

2. The Troxler effect must occur

3. Retinal neurons must be able to respond with gaps in their firing pattern

4. All of the other answers are correct

How does this measure of temporal acuity (CFF) change under different conditions (changes in the stimulus dimensions listed in Chapter 1)?

First: stimulus luminance (intensity)

Important Stimulus Dimensions

intensity

wavelength

size

exposure duration

frequency

shape

relative locations of elements of the stimulus

cognitive meaning

In addition,(NOT stimulus Dimensions!)

location on the subject’s retina

light adaptation of the subject’s visual system

CFF is directly proportional to the log of stimulus luminance (Ferry -Porter Law)

CFF = k log L + b where k is the slope of the function , b is a constant, and L is the luminance of the flickeringstimulus

Log Retinal IIluminance (Td)

-1 0 1 2 3 4

Critical FlickerFrequency (Hz)

0

10

20

30

40

50642 nm at the fovea

Note: if the luminanceof the stimulus increasesby one log unit, so does the retinal illuminance

demo

1) Find CFF

2) Raise intensity (luminance) by 1 log unit. The more intense stimulus is below CFF (flicker is seen).

3) Have to increase the flicker rate to again find CFF.

Log Retinal IIluminance (Td)

-1 0 1 2 3 4

Critical FlickerFrequency (Hz)

0

10

20

30

40

50642 nm at the fovea

1

2

3

The Ferry-Porter law holds at all eccentricities. The slope is steeper

in the periphery. At high luminance, CFF is higher in the periphery

than at the fovea.

Log Retinal Illuminance for 0oand 3

o (td)

-1 0 1 2 3 4 5 6 7

Critical Flicker Frequency (Hz)

0

20

40

60

80

100

Log Retinal Illuminance for 10o-85

o (td)

-4 -3 -2 -1 0 1 2 3 4

0o

3o

10o

35o

65o

85o

Retinal Eccentricity (deg)

0 20 40 60 80 100

Critical Flicker Frequency (Hz)

0

20

40

60

80

100

0.25 2.5

25 250

2500

Retinal Illuminance (td)

The CFF is highest in the midperipheral retina at high luminance,

but nearly constant across the retina at low luminance.

This is why you can see flicker on some PC monitors if you look slightly to the side

CFF increases

In d

irect

pro

portio

n to...

In th

e per

iphe

ry a

t all.

..

In re

spons

e to

the

Br...

None

of the

abo

ve

0% 0%0%0%

1. In direct proportion to the log of the stimulus luminance

2. In the periphery at all luminance levels

3. In response to the Brücke-Bartley phenomenon

4. None of the above

CFF is directly proportional to the logarithm of the area of the flickering stimulus (the

Granit-Harper Law)

Second: area (size)

Demo, since I haven’t found a good figure showing this relationship

CFF = k logA + b

Where k and b are constants and A is the area of the flickering stimulus

Demo – Granit-Harper

1) Find CFF

2) Increase stimulus area by 1 log unit. The more intense stimulus is below CFF (flicker is seen).

3) Have to increase the flicker rate to again find CFF.

Chapter 7 – Temporal Factors in Vision

Main points so far:

1) CFF is a measure of temporal acuity – analogous to VA (how small a temporal interval can you detect – in time)?

2) CFF increases linearly with log stimulus luminance (Ferry-Porter Law)

3) CFF increases linearly with log stimulus area (Granit-Harper)

You will not be responsible for the material starting on page 188, “flicker sensitivity increases….”

and including all of page 189 and 190 (Figs. 7.7 and 7.8).

You will be responsible for material starting again on page 191, “Temporal Contrast Sensitivity”




Temporal Summation

Masking


Contrast, modulation and amplitude

The contrast of a temporal sine wave is defined the same way as the contrast of a spatial sine wave

grating:

Contrast = (Lmax Lmin)/( Lmax + Lmin)

In Figure 7-1, Lmax is 300, Lmin is 100, so contrast = (200)/(400) = 0.5 Another term, modulation

(abbreviated as m),is sometimes used for sine-wave flicker, and may be used interchangeably with

contrast. As illustrated in Figure 7-1, Lmax is the maximum luminance of the flicker, and Lmin is the

minimum luminance. Lmax and Lmin are symmetrically arranged around the mean or average luminance,

defined as:Mean Luminance = L

m = ( Lmax + Lmin)/2

Hence, contrast or modulation can also be expressed as:

Contrast = m = (Lmax Lm)/ L

m

In addition, Lmax - Lm is also called the amplitude of the wave, and, therefore,

Contrast = modulation = amplitude /Lm

Referring again to the sine wave at the bottom of Figure 7-1, the

mean luminance is 200 units, the amplitude is 100, and the contrast (modulation) therefore is 0.5. As

was the case for spatial sine-wave gratings, contrast sensitivity is defined as the inverse of the threshold

contrast.

Temporal CSFs have several features in common with spatial CSFs:

band pass shape, cutoff high frequency indicating the acuity limit, and a low frequency

rolloff.

This is like Figure 6.9 in the spatial domain

Frequency (Hz)

2 5 10 20 50 100

ContrastSensitivity

1

2

5

10

20

50

100

200

Threshold Contrast

1

0.5

0.2

0.1

0.05

0.02

0.01

0.005

Retinal Illuminance (Td)

9300 850 77 7.1 0.65 0.06

Temporal CSF Demo

http://psy.ucsd.edu/~sanstis/TMTF.html



Change in the temporal CSF with luminance:

As luminance decreases,

the peak contrast sensitivity becomes lower

the cutoff high temporal frequency decreases (Ferry-Porter law)

peak contrast sensitivity occurs at lower temporal frequency

the low temporal frequency rolloff disappears

Frequency (Hz)

2 5 10 20 50 100

ContrastSensitivity

1

2

5

10

20

50

100

200

Threshold Contrast

1

0.5

0.2

0.1

0.05

0.02

0.01

0.005


9300 850 77 7.1 0.65 0.06

The temporal contrast sensitivity function

Is th

e bo

undary

bet

...

Has

a p

eak

contra

st a

...

Is a

mea

sure

of t

empo

r..

Bec

omes

more

ban

d...

0% 0%0%0%

1. Is the boundary between contrasts you can see and ones you cannot see

2. Has a peak contrast at around 1 Hz at high mean luminance

3. Is a measure of temporal acuity

4. Becomes more bandpass as the mean luminance is decreased

The center-surround interactions of retinal neurons may account for a low

frequency roll-off in temporal CSF of individual neurons

Actually, there is a mid-temporal frequency enhancement of sensitivity

The delayed arrival of the surround signal, relative to the center signal can cause the surround to add with the center at some temporal frequencies

The delayed arrival of the surround signal, relative to the center signal can cause the surround to add with the center at mid-range temporal frequencies

Temporal CSFs have several features in common with spatial CSFs:

band pass shape, cutoff high frequency indicating the acuity limit, and a low frequency

rolloff.

This is like Figure 6.9 in the spatial domain

Frequency (Hz)

2 5 10 20 50 100

ContrastSensitivity

1

2

5

10

20

50

100

200

Threshold Contrast

1

0.5

0.2

0.1

0.05

0.02

0.01

0.005


9300 850 77 7.1 0.65 0.06

1. Artificially increased IOP produces reduced temporal CSF (but no effect on CFF)

2. Temporal CSF is reduced with glaucoma and ocular hypertension

• Glaucoma - Frequency-doubling perimeter measures contrast threshold for 0.25 c/deg grating flickering at 25 Hz (mediated by MY [nonlinear magno] cells?)

3. Eyes at risk for exudative (wet) AMD show reduced sensitivity at 5 - 40 Hz (5 Hz & 10 Hz alone discriminate from healthy eyes)

Importance? Early diagnosis can lead to earlier treatments

The temporal CSF is a useful measure for diagnosing retinal disorders

The low temporal frequency rolloff of the temporal CSF

Is re

ally

a “

mid

-tem

por..

Bec

omes

more

pro

m...

Hel

ps cr

eate

Mac

h b...

Is re

late

d to th

e cu

tof..

.

0% 0%0%0%

1. Is really a “mid-temporal frequency enhancement produced by the longer latency of the receptive field surround

2. Becomes more prominent at low mean luminance levels

3. Helps create Mach bands4. Is related to the cutoff high

temporal frequency




Temporal Summation (Bloch’s Law & Broca-Sulzer)

Masking


Flash Duration (s)

0.001 0.01 0.1 1 10 100

4

5

6

7

8

9

Log Threshold Luminance(quanta/s/deg2)

Stimulus area = 0.011 deg2

Log Background Intensity

7.83 5.94 4.96 3.65 No Background

Fig. 2.5

Bloch’s Law holds for durations shorter than the critical duration

L x t = C Eq. 7.7

where L is the threshold luminance of the flash, t is its duration, and C is a constant

Remember: luminance (L) is directly proportional to the number of quanta (Q) in a flash

and inversely proportional to the duration (t) and area (A) of the flash, or

L = Q / t x A

C x duration area duration x

quanta Eq. 2.6

There is a constant # of quanta in a threshold flash as L decreases

Temporal Summation and Bloch's Law

When a brief flash is used to determine the threshold intensity, the visual system does not distinguish the “temporal shape” of the flash if the flash duration is less than the “critical duration”

Part A – threshold measures

Numberof

Quanta

TimeCritical Duration

TimeCritical Duration

BA

Flash Duration (msec)

1 10 100

Log ThresholdLuminance

1 10 100

Log Threshold Luminance x Time

Bloch’s Law holds

Bloch’s Law holds

“Holds” means that Bloch’s Law accounts for the threshold values

Two ways to show Bloch’s Law: L x t = C

Bloch's law is a consequence of the temporal filtering properties of vision.

But I will not hold you responsible for this section

Bottom of 198 & top of 199

Bloch's law is a consequence of the temporal filtering properties of vision.

F

F+3F

F+3F+5F

F+3F+5F+7F

F+3F+5F+7F+9F

F+3F+5F+7F+9F+11F

F

3F

5F

7F

9F

11F

Fourier Synthesis: can construct complexwaveforms by adding together simple ones

Horizontal Position (deg)

0.0 0.2 0.4 0.6 0.8 1.0

ALuminance

Spatial Frequency (cycles/deg)1 3 5 7 11 17 25

BRelativeContrast

0.1

1

10

100

1000


CRelativeSensitivity

0.01

0.1

1

10


DRelativeContrast

0.1

1

10

100

1000

Horizontal Position (deg)0.0 0.2 0.4 0.6 0.8 1.0

EBrightness

Flashes of various durations shorter than the critical duration all have the same temporal frequency spectrum. Flashes longer than the critical duration contain less contrast at intermediate temporal frequencies, after filtering through the temporal CSF and are therefore less visible. Thus, more quanta are need to be added to bring them up to threshold.

The critical duration for a brief flash against a background decreases as the luminance

of a background light or area of the flash increases

Log Flash Duration (msec)

0 1 2 3

Log Threshold RetinalIlluminance (Td)

-2

-1

0

1

2

3

4

2500 3400 456 115 21 9.5 1.9 0 0.43

Background Luminance (Td)

fovea 1o

Critical duration also depends on stimulus area. As

the area of the flash is increased, the critical duration

decreases.

When the stimulus diameter is small (1.5 - 2 min

arc), Bloch's Law holds for flash durations up to

around 0.10sec (100 msec).

If the test flash diameter increases to approximately

5 deg., Bloch's Law only holds for flashes up to about

30 msec in duration.

For flash durations less than the critical duration, Bloch’s Law holds and

1. The flash cannot be seen when it is above threshold

2. The number of quanta in a threshold flash is the same for different flash durations

3. L x C = t4. None of the above




Temporal Summation (Bloch’s Law & Broca-Sulzer)

Masking


Supra-threshold flashes of a certain brief duration appear brighter than longer and

shorter flashes of the same physical intensity (Broca-Sulzer effect)

170

16.2 lux

32.4 lux

64.5 lux

126 lux

170 lux

126

64.5

32.416.2

0.0

1

0.2

0.5

0.2

5

0.1

25

0.1

0.0

62

0.0

46

0.0

37

Flash Duration (sec)

Co

mp

ara

tive

Brig

htn

ess

700

600

500

400

300

200

100

0

Part B – above-threshold brightness

Broca

Neural Explanation

•Intense stimuli produce photoreceptor overshoot

•This produces (via the bipolar cells) an initial burst of action potentials in the ganglion cells

•Brightness is related to the firing rate of the cells (spikes/second)

•For long flashes, the firing rate after the initial burst signals the brightness

•For brief flashes, only the initial burst occurs, so the only information the neurons in central structures can use is a high firing rate, which makes the flash appear brighter than when it is long.

Neural explanation of the Broca-Sulzer effect

Note: the photoreceptor membrane potentials are upside down (negative is up on the graph) to demonstrate the similarity in shape to the Broca-Sulzer effect.

Neural Explanation

•Intense stimuli produce photoreceptor overshoot

•This produces (via the bipolar cells) an initial burst of action potentials in the ganglion cells

•Brightness is related to the firing rate of the cells (spikes/second)

•For long flashes, the firing rate after the initial burst signals the brightness

•For brief flashes, only the initial burst occurs, so the only information the neurons in central structures can use is a high firing rate, which makes the flash appear brighter than when it is long.




Temporal Summation

Masking (Temporal interactions between visual stimuli)


Temporal Interactions between Visual Stimuli

Masking is any situation in which the detection of a visual stimulus is reduced by

another stimulus presented before, during, or after the target stimulus.

The effects of a masking stimulus may continue forward, after its

cessation, and backwards, before its onset

1) masking of light by light, 2) masking of a pattern by light, and 3) masking of a pattern

by a pattern.

Test Field Onset Time (msec)

0 250 500 750 0

Log Threshold Test Field Energy

Test Off

Test On

Mask Off

Mask On

-4

-3

-2

-1

0Masking Stimulus (1.38o)

Test Stimulus (0.36o)

MaskingForwardmasking

Backwardmasking

Backward Masking might remind you of Early Dark Adaptation

Test flash

Masking flash

Test Field Onset Time (msec)

0 250 500 750 0

Log Threshold Test Field Energy

Test Off

Test On

Mask Off

Mask On

-4

-3

-2

-1

0Masking Stimulus (1.38o)

Test Stimulus (0.36o)

MaskingForwardmasking

Backwardmasking

Backward Masking might remind you of Early Dark Adaptation

Simultaneous and forward masking are signal detection problems

Target pulse aloneS

pik

es p

er s

eco

nd 60

Mask follows target by 300 milliseconds

Sp

ikes

per

sec

on

d 60

0

Mask pulse alone

Sp

ikes

per

sec

on

d 60

0

0


Spi

kes

per

seco

nd 60

0


Spi

kes

per

seco

nd 60

0


Spi

kes

per

seco

nd 60

0


Spi

kes

per

seco

nd

Time (milliseconds)

60

00 1000 2000

Time (milliseconds) 0 1000 2000

Backward masking may be explained by the response latency and duration of the test flash

Masking effects do not require spatial coincidence of test and masking stimuli; they

may occur when the test and mask are spatially separated by as much as 3 degrees

Masking effects may occur when the test and mask are spatiallyseparatedmetacontrast (backwards) and paracontrast (forwards)are masking in which the test flash and masking flash do not overlap spatially on the retina

This suggests that the same cells must be stimulated by the edges of both stimuli to obtain metacontrast

When the gap between the stimuli becomes large enough, different populations of retinal neurons are stimulated by the test and masking flashes. Any masking has to occur upstream in the visual pathway, where receptive fields get larger

Masking

• Masking is any situation in which the detection of a visual stimulus is reduced by another stimulus presented before, during, or after the target stimulus.

• Metacontrast (backwards masking with physically-separated stimuli) and paracontrast (forward masking with physically separated stimuli)

• Dichoptic masking – masking where the two stimuli are presented to different eyes

Dichoptic masking - A masking stimulus presented to one eye affects vision of a test

stimulus at a corresponding retinal location in the other eye

Cannot occur until inputs from the two eyes meet at abinocular cell in V1 or later

Saccadic suppression

Saccadic suppression is defined as a reduction in sensitivity to visual stimuli that occurs before, during and after a saccade

(Look at your eyes in a mirror and try to see them move when you make a saccade)

Decreased sensitivity (increased threshold) to visual stimuli

occurring before, during, and after saccadic eye movements

100

-120 -80 -40 0 40 80 120 160

80

60

40

20

0

100

80

60

40

20

0

Visual suppression

Pupil suppression

Pu

pil

Re

sp

on

se

(p

erc

en

t)

Vis

ua

l R

es

po

ns

e (

pe

rce

nt)

Time of Flash (msec)

Eyes moving

But you can see the strobe lights atop Red Mountain if you time your saccade just right

Masking includes

1. any situation in which the detection of a visual stimulus is reduced by another stimulus presented before, during, or after the target stimulus

2. Paracontrast

3. Dichoptic masking

4. All of the above




Temporal Summation

Masking


Motion is a continuous change in an object’s location as a function of time

Three reasons motion detection is important:

•detect moving objects against a background (see edges)

•detect own motion through the environment

•determine 3-D shape (crudely)

Demo – shape from motion

(If you can’t see the edges, you can’t see the object)

http://www.biomotionlab.ca/Demos/BMLwalker.html

http://www.biomotionlab.ca/Demos/BMLwalker.html

Real Motion:Motion involve an image changing its location on the retina

Contrast with smooth pursuit (moving the eyes smoothly). This prevents the image from changing its location on the retina. We are not studying smooth pursuit.

There is an upper limit to our ability to see motion – stimuli can be moving “too fast to see”

It turns out that the reason is that rapidly moving images have a temporal frequency that is too high for our visual system to detect (frequency is above the temporal high-frequency cutoff).

To understand this – need to look at movement from the point of view of an individual retinal neuron.

From the viewpoint of any one cell in the retina, motion is a change in luminance that occurs at a rate that depends on the speed with which the object moves andon the spatial frequency composition of the object

•A high spatial frequency grating moving at constant velocity (degrees per second) has a faster temporal frequency than a lower spatial frequency moving at the same velocity

This grating moved one full cycle

Motion involves interactions of both spatial frequency and temporal frequency

•Now, a lower spatial frequency (about half of the first one)moving at the same velocity (degrees per second).

It has lower temporal frequency (cycles per second) at a given spot

This grating moved about ½ cycle.

Measured at the green dot (symbolizing a receptive field), it has a temporal frequency (flicker rate) about half of the higher spatial frequency

Can determine the temporal frequency of a drifting grating by multiplying its spatial frequency times its velocity in degrees per second

A 3 cycle/deg grating moving 10 deg/sec has a temporal frequency of 30 Hz; 30 cycle/deg =300 Hz

Temporal Frequency (cycles/s)

0.01 0.1 1 10 25 50

Spatial Frequency (cycles/deg)

0.01 0.1 1 10 25 50

ContrastSensitivity

1

10

100

10000110100800

Velocity (deg/s)A B

As object velocity increases, spatial CSF shifts to lower spatial frequencies; temporal CSF remains constant

How fast a velocity can you see moving?

The limiting factor in motion detection is the temporal resolution of the visual system.

If you present a very, very low spatial frequency (and high contrast) can see motion of several thousand degrees per second

The ability to see rapidly-moving (high velocity) objects

1. Is limited by the temporal frequency

2. Occurs only in the visual cortex

3. Is set by the velocity of the objects

4. Cannot be measured

Apparent Motion:Apparent motion is the perception of real motion that can be produced when a stimulus is presented discontinuously.

Phi phenomenonhttp://www.yorku.ca/eye/balls.htm

http://www.yorku.ca/eye/balls.htm

http://www.yorku.ca/eye/balls.htm

Apparent Motion:Apparent motion is the perception of real motion that can be produced when a stimulus is presented discontinuously.

The “rules” for producing apparent motion are the same as for real motion: the optimal stimulus duration and spacing is the same as would occur if a real object moved.

Real vs. Apparent Motion Motion sampled stroboscopically

appears like real motion due to the

insensitivity of vision to high

temporal and spatial frequencies

Velocity (deg/sec)

0.2 0.5 1 2 5 10 20 50 100

Optimal Time (msec)

20

50

100

200

0.2 0.5 1 2 5 10 20 50 100

Optimal Distance (arc min)

2

5

10

20

50

100

200

Burr and Ross, 1982Van Deenna and Kimurama, 1982Nakayama and Silverman, 1984Kelly, 1979

To produce optimal apparent motion of 10 degrees per second, need each spot to be about 25’ apart and be on for about 35 msec. A real object, traveling at 10 degrees per second would move 21’ in the same time

In order to make apparent motion look like real motion

1. You have to “fool” some of the neurons all of the time

2. You need a string of lights

3. You need real motion4. You need to present

the stimuli with the same separation and duration as would occur with real motion

Detection of motion and sensitivity to direction of motion is achieved in hierarchic

fashion in Areas V1 of the striate and middle temporal region of the cortex

Newsome and colleagues sampled the activity of neurons in area MT

Each cell has a receptive field that responded to motion in some location in the visual field (some retinal location). Each neuron was direction selective; it had an optimal direction (most spikes per second) and a null direction (fewer spikes per second).

Stimuli with a range of correlation of the motion of the spots were used to determine threshold amount of correlation for the monkey, and also the threshold for neurons in the monkey’s area MT (in a two-alternative, forced-choice situation).

Using signal detection theory, a “neurometric” function could be produced for each neuron and compared with the monkey’s psychometric function

Spikes per Trial0 100

20

20

Number ofTrials

20

Non-preferred directionPreferred direction

Correlation - 12.8%

Correlation - 3.2%

Correlation - 0.8%

A

B

Correlation (%)0.1 1 10 100

PercentCorrect

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Psychometric FunctionNeurometric Function

Frequency ofOccurence

0

1

2

3

4

5

6

7

Mean of Noise

Number of Action Potentials in 50 msec Period

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

0

1

2

3

4

5

6

7

Mean of Noise + Signal

Overlap: PossibleConfusion

Maintained Discharge (Noise)Distribution

Maintained Discharge (Noise) +Response to Flash (Signal)

Distribution

A

B

Srimulus AbsentStimulus Present

d'=1.5d'=1.0d'=0.5

A

B

C

ROC Curve

Using signal detection theory, a “neurometric” function could be produced for each neuron and compared with the monkey’s psychometric function

Spikes per Trial0 100

20

20

Number ofTrials

20

Non-preferred directionPreferred direction

Correlation - 12.8%

Correlation - 3.2%

Correlation - 0.8%

A

B

Correlation (%)0.1 1 10 100

PercentCorrect

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Psychometric FunctionNeurometric Function

Real & apparent motion seem to be detected by neurons in the parietal (MT) “stream”

Threshold Ratio (neuron/behavior)

0.1 1 10

Number ofNeurons

0

5

10

15

20

The psychometric function for the monkey was matched well by direction-selective neurons in area MT.

Monkey more sensitive than the neuronNeuron more sensitive than the monkey

The monkeys’ “neurometric function”

1. Did not match the psychometric function

2. Could not be accurately estimated

3. Closely matched the psychometric function

4. None of the above

Adapting to one direction of motion can produce a motion aftereffect when the movements stops (the “waterfall illusion”)

May be due to neurons in MT

Waterfall Illusion http://www.yorku.ca/eye/mae.htm

http://www.yorku.ca/eye/mae.htm

http://www.yorku.ca/eye/mae.htm




Temporal Summation

Masking


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X The Troxler effect: the fading of a large stimulus with blurred edges presented in the peripheral visual field